Sapphire Mode-locked Laser Research Articles

Unveiling out-of-loop attosecond timing jitter precision in Ti:sapphire mode-locked lasers with an optical heterodyne technique.

The out-of-loop timing jitter exhibited in free-running Ti:sapphire mode-locked lasers with attosecond resolution is demonstrated using an optical heterodyne technique. To assess the feasibility of the experiment and discrimination signal properties, numerical simulations were conducted for Ti:sapphire mode-locked lasers. For accurately characterizing the genuine phase noise exhibited by Ti:sapphire mode-locked lasers, out-of-loop measurements were conducted, and a straightforward yet improved optical heterodyne setup was employed, allowing simultaneous low-bandwidth locking and out-of-loop timing jitter measurements with two Ti:sapphire mode-locked lasers. The out-of-loop phase noise floor for a single mode-locked laser reaches -203.47 d B c/H z, assuming a 10 GHz carrier frequency. Additionally, the out-of-loop integrated timing jitter is 11.9 a s from 10 kHz to the Nyquist frequency (50 M H z).

Optical frequency divider: Capable of measuring optical frequency ratio in 22 digits

Recent advances in optical frequency standards and optical frequency combs (OFCs) have drawn wide attention since by transforming other quantities into frequency metrology, a higher measurement sensitivity or accuracy can be achieved. Among them, the search for dark matter, tests of relativity, and detection of gravitational wave anticipate even more precise frequency ratio measurement of optical signals, which challenges the state-of-the-art optical frequency standards and OFCs. Here, we report an optical frequency divider (OFD) based on a Ti:sapphire mode-locked laser, which can realize ultraprecise optical frequency ratio measurements and optical frequency division to other desired frequencies. The OFD is based on an OFC frequency-stabilized to a hydrogen maser, whose frequency noise in optical frequency division is subtracted via the transfer oscillator scheme. An optically referenced radio frequency time-base is introduced for the fine-tuning of the divisor and the reduction in division noise. Using the OFD, the frequency ratio between the fundamental and its second harmonic of a 1064 nm laser is measured with a fractional uncertainty of 3 × 10−22, nearly five times better than previous results. Meanwhile, we also report the ability to transport between laboratories, the long-term operation, and the multi-channel division of the OFD.

Long-term frequency-stabilized optical frequency comb based on a turnkey Ti:sapphire mode-locked laser

We report a long-term frequency-stabilized optical frequency comb at 530–1100 nm based on a turnkey Ti:sapphire mode-locked laser. With the help of a digital controller, turnkey operation is realized for the Ti:sapphire mode-locked laser. Under optimized design of the laser cavity, the laser can be mode-locked over a month, limited by the observation time. The combination of a fast piezo and a slow one inside the Ti:sapphire mode-locked laser allows us to adjust the cavity length with moderate bandwidth and tuning range, enabling robust locking of the repetition rate (fr) to a hydrogen maser. By combining a fast analog feedback to pump current and a slow digital feedback to an intracavity wedge and the pump power of the Ti:sapphire mode-locked laser, the carrier envelope offset frequency (fceo) of the comb is stabilized. We extend the continuous frequency-stabilized time of the Ti:sapphire optical frequency comb to five days. The residual jitters of fr and fceo are 0.08 mHz and 2.5 mHz at 1 s averaging time, respectively, satisfying many applications demanding accuracy and short operation time for optical frequency combs.

Practical two-photon-absorption cross sections and spectra of eosin and hematoxylin.

We experimentally investigate the two-photon absorption cross sections and spectra of eosin and hematoxylin for applications in nonlinear microscopy. The experiments are carried out on pure samples of the two dyes in DI-water solvent with different concentrations, in the typical range employed in standard staining procedures. Nonlinear fluorescence is excited by a line-shaped beam emitted by a Ti:Sapphire mode-locked laser in the wavelength range from 740 to 880 nm and is detected through a microscope setup. The two-photon absorption spectral response is systematically analyzed and discussed. Finally, the staining is applied on biological tissue samples, which are imaged by two-photon microscopy. Our results show that the employed dyes are fully suitable for applications in nonlinear imaging.

Carrier-envelope offset frequency stabilization of a 100 fs-scale Ti:sapphire mode-locked laser for quantum frequency comb generation

The carrier-envelope offset (CEO) frequency stabilization of a Ti:sapphire mode-locked laser with a Fourier-transform-limited pulse width of 100 fs-scale has been successfully realized. With the home-built f-2f interferometer, the CEO beat signal has been measured with a signal-to-noise ratio (SNR) as high as 40 dB under a resolution bandwidth of 100 kHz, which means that the coherence of the octave-spanning supercontinuum (SC) is well retained, although the evaluated pulse soliton order (N ≈ 23) completely violate the previous theoretical limitation that N should be smaller than 16. Furthermore, a stabilized CEO frequency is achieved with residual phase noise suppressed to 660 mrad (integrated from 1 Hz to 100 kHz) and an Allan deviation relative to the optical carrier of 7.6 × 10−18 at 1 s integration time. This realization constructs a solid foundation for further generation of quantum frequency comb needed for ultra-sensitive optical timing measurement applications.

Gripe water-mediated green synthesis of silver nanoparticles and their applications in nonlinear optics and surface-enhanced Raman spectroscopy

In the present study, silver nanoparticles were synthesized from aqueous silver nitrate through a simple, eco-friendly and ‘green’ method using gripe water as reducing as well as stabilizing agent. Control over the dispersity of silver (Ag) nanoparticles was attained by altering the synthesis process. The size and morphology of the particles were perceived using high-resolution transmission electron microscope and the surface plasmon resonance of the prepared nanoparticles was observed by UV–VIS spectrum. Herein, we report the nonlinear optical behavior and surface-enhanced Raman spectroscopy of silver nanoparticles with different particle size and dispersity. The nonlinear optical behavior was studied by single beam Z-scan technique using tunable Ti: Sapphire mode-locked femtosecond laser as source. The nonlinear optical parameters such as the nonlinear refractive index, nonlinear absorption coefficient β and the third-order nonlinear susceptibility χ3 of the prepared Ag nanoparticles were obtained for various wavelengths by tuning the wavelength of the laser from 700 to 950 nm. Surface-enhanced Raman spectroscopy (SERS) is an inspiring phenomenon especially in the case of silver nanoparticles. The as-synthesized silver nanoparticles show huge enhancements in the order of 109 in the Raman spectrum of rhodamine 6G dye.

Supercontinuum generation in photonic crystal fibre by Ti:sapphire femtosecond laser

We present an experimental study of supercontinuum generation in a highly non-linear photonic crystal fibre with a zero-dispersion wavelength of 780 nm pumped by femtosecond pulses from a Ti:sapphire mode-locked laser. Supercontinuum light with a spectrum ranging from 500 to 1100 nm has been generated in the photonic crystal fibre with an average core radius of 1·8 μm. Those optical pulses have a duration of 35 fs, a repetition rate of 350 MHz and an average power of 360 mW. Moreover, different supercontinuum can be generated by changing the input power and the polarisation direction.

Generation of high power pulsed terahertz radiation using a plasmonic photoconductive emitter array with logarithmic spiral antennas

An array of 3 × 3 plasmonic photoconductive terahertz emitters with logarithmic spiral antennas is fabricated on a low temperature (LT) grown GaAs substrate and characterized in response to a 200 fs optical pump from a Ti:sapphire mode-locked laser at 800 nm wavelength. A microlens array is used to split and focus the optical pump beam onto the active area of each plasmonic photoconductive emitter element. Pulsed terahertz radiation with record high power levels up to 1.9 mW in the 0.1–2 THz frequency range is measured at an optical pump power of 320 mW. The record high power pulsed terahertz radiation is enabled by the use of plasmonic contact electrodes, enhancing the photoconductor quantum efficiencies, and by increasing the overall device active area, mitigating the carrier screening effect and thermal breakdown at high optical pump power levels.

Long-term stable passive synchronization between two-color mode-locked lasers with the aid of temperature stabilization

We demonstrate long-term stable passive synchronization between two-color Ti:sapphire (master) and Yb-doped fiber (slave) mode-locked lasers in the master-slave configuration. Active temperature stabilization suppresses the repetition fluctuation of the slave laser, and with the aid of temperature stabilization in combination with simple repetition locking of the master laser, long-term stable synchronization as long as 6 h was realized. The repetition rates of both lasers are locked in submillihertz precision. A timing jitter of 0.75 fs was obtained at a detection bandwidth of 350 kHz.

Demonstration of a Ti:sapphire mode-locked laser pumped directly with a green diode laser

We demonstrate a Ti:sapphire laser pumped directly with a green diode laser. A single 1 W InGaN diode laser operating at 518 nm is used as the pump source. Pulse durations as short as 62 fs and average output powers of up to 23.5 mW are obtained with chirped-mirror-based dispersion compensation and a semiconductor saturable absorber mirror (SESAM).

Multiheterodyne interference spectroscopy using a probing optical frequency comb and a reference single-frequency laser

We discuss multiheterodyne interference between the laser beams of optical frequency-comb (FC) and single-frequency (SF) lasers. The beat signal generated by the interference contains a dc component, discrete-frequency elements with the same frequency interval generated by interference between the two elements of the FC laser, and elements generated by the interference between the FC and the SF lasers. The beat-signal elements between them can be used for absorption spectroscopy around the SF-laser frequency, which is called frequency-comb interference spectroscopy (FCIS). In this paper, we compare the beat-signal spectrum predicted through theoretical descriptions of multiheterodyne interference with experimental results to improve understanding of FCIS and similar techniques. Our FCIS experiments use a probing Ti:sapphire mode-locked laser, which is the beam experiencing absorption, and a reference diode laser. We also discuss the potential of this method to measure the small shift in absorption profiles using the measurement results of argon metastable atoms.

Photonic microwave generation with high-power photodiodes

We utilized and characterized high-power, high-linearity modified unitraveling carrier (MUTC) photodiodes for low-phase-noise photonic microwave generation based on optical frequency division (OFD). When illuminated with picosecond pulses from a repetition-rate-multiplied gigahertz Ti:sapphire modelocked laser, the photodiodes can achieve a 10 GHz signal power of +14 dBm. Using these diodes, we generated a 10 GHz microwave tone with less than 500 attoseconds absolute integrated timing jitter (1 Hz-10 MHz) and a phase noise floor of -177 dBc/Hz.We also characterized the electrical response, amplitude-to-phase conversion, saturation, and residual noise of the MUTC photodiodes.

Synchronized time-lens source for coherent Raman scattering microscopy

We use the time-lens concept to demonstrate a new scheme for synchronization of two pulsed light sources for biological imaging. An all fiber, 1064 nm time-lens source is synchronized to a picosecond solid-state Ti: Sapphire mode-locked laser by using the mode-locked laser pulses as the clock. We demonstrate the application of this synchronized source for CARS and SRS imaging by imaging mouse tissues. Synchronized two wavelength pulsed source is an important technical difficulty for CARS and SRS imaging. The time-lens source demonstrated here may provide an all fiber, user friendly alternative for future SRS imaging.

Square nanosecond Yb- and Er-doped fiber lasers passively synchronized to a Ti:sapphire laser based on cross-absorption modulation

Robust synchronization was achieved for Ti:sapphire, Yb- and Er-doped fiber mode-locked lasers in a master-slave configuration. Square nanosecond pulses were generated in the long-cavity fiber lasers, while synchronization was maintained owing to resonance-enhanced cross-absorption modulation in an additional Er-doped fiber, allowing for a cavity mismatch tolerance of 8.2 and 3.6 cm, respectively. This is the highest tolerable cavity-length difference ever obtained for synchronized mode-locked lasers.

High-order dispersion-resulted tunable multiwavelength femtosecond laser

Resonant dispersive waves generated by high-order dispersion in a Ti:sapphire mode-locked solitary laser are investigated and experiments are found to be in good agreement with theoretical analysis. Both theory and experiment show that the wavelength differences can be tuned in a large range via changing the cavity dispersion. This simple technique can be applied to the fields where tunable femtosecond pulses at multiple wavelengths are needed. The described mechanism may be applied in other systems where solitons are known to exist.

Direct comparison of optical frequency combs using a comb-injection-lock technique

This paper demonstrates a direct comparison of optical frequency combs (OFCs) with different repetition rates without a stable intermediate laser using a single-mode comb-injection-lock technique. Two OFCs based on Ti:Sapphire mode-locked lasers were compared utilizing a single-mode diode laser for the selection and the amplification of one mode of an OFC by comb-injection, which makes the direct comb comparison possible. The frequencies of the two combs were found to agree within 0.019 Hz at 352 THz with the uncertainty of 0.25 Hz (7.1 x 10(-16) ). This is one of the best results among the comparisons of combs referenced to a microwave frequency. This technique simplifies the comb comparison utilities and can be applied even when repetition rates differ.

High Harmonic Frequency Combs for High Resolution Spectroscopy

We generated a series of harmonics in a xenon gas jet inside a cavity seeded by pulses from a Ti:sapphire mode-locked laser with a repetition rate of 10.8 MHz. Harmonics up to 19th order at 43 nm were observed with plateau harmonics at the microW power level. An elaborate dispersion compensation scheme and the use of a moderate repetition rate allowed for this significant improvement in output power of the plateau harmonics of 4 orders of magnitude over previous results. With this power level and repetition rate, high-resolution spectroscopy in the extreme ultraviolet region becomes conceivable. An interesting target would be the 1S-2S transition in hydrogenlike He+ at 60 nm.

Femtosecond frequency comb and optical frequency measurement of 532 nm Nd:YAG laser

In this paper we report on the Ti:sapphire mode-locked femtosecond laser based optical frequency comb apparatus and developed by NIM its application in optical frequency measurement of the I2 stabilized 532nm Nd∶YAG laser installed in NIM. The measurement result of 127I2 R(56)32-10 hyperfine transition is 563260223512991±20 (Hz), with a relative frequency uncertainty of 3.6×10-14. To our knowledge，this result is the first optical frequency measurement in China which is referenced directly to the Cs microwave frequency standard.

Solid-state carrier-envelope-phase noise measurements with intrinsically balanced detection

We use interference between single- and two-photon photocurrent generation pathways in a semiconductor to measure the out-of-loop carrier-envelope-phase noise of a stabilized Ti:sapphire modelocked laser. This solid-state measurement technique exhibits no significant amplitude/phase coupling, adds no measurable phase noise compared to the standard self-referencing technique, and requires few optical components. The method features a built-in balanced detection mechanism that is particularly appealing for dc carrier-envelope-phase measurements.

Colloidal metal particles as probes of nanoscale thermal transport in fluids

We investigate suspensions of 3--10 nm diameter Au, Pt, and AuPd nanoparticles as probes of thermal transport in fluids and determine approximate values for the thermal conductance G of the particle/fluid interfaces. Subpicosecond $\ensuremath{\lambda}=770\mathrm{nm}$ optical pulses from a Ti:sapphire mode-locked laser are used to heat the particles and interrogate the decay of their temperature through time-resolved changes in optical absorption. The thermal decay of alkanethiol-terminated Au nanoparticles in toluene is partially obscured by other effects; we set a lower limit $Gg20\phantom{\rule{0ex}{0ex}}{\mathrm{MW}\mathrm{m}}^{\ensuremath{-}2}{\mathrm{K}}^{\ensuremath{-}1}.$ The thermal decay of citrate-stabilized Pt nanoparticles in water gives $G\ensuremath{\approx}130{\mathrm{MW}\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0ex}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}.$ AuPd alloy nanoparticles in toluene and stabilized by alkanethiol termination give $G\ensuremath{\approx}5{\mathrm{MW}\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0ex}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}.$ The measured G are within a factor of 2 of theoretical estimates based on the diffuse-mismatch model.